THE FORCES OF AIR AND STEAM.
QUESTION 7. _What is meant by the pressure of the air?_
_Answer._ It is the pressure exerted by the weight of the air on
every point with which it is in contact. The globe of the earth is
surrounded by a layer of air about 50 miles thick, and, like every
other substance, the air possesses weight, and hence presses upon every
object with which it is in contact.
QUESTION 8. _How can it be shown that the air possesses weight?_
_Answer._ By weighing a flask when it is filled with air, and again
when the air is exhausted from it. In the latter condition the weight
of the flask will be found to be sensibly less than it was when full of
air, showing that the air which the flask contained when it was first
weighed increased its weight.
QUESTION 9. _Why do we not feel this pressure on our bodies?_
_Answer._ Because the air surrounds us on all sides, and presses
just as much in one direction as it does in another, so that the
pressures in different directions just balance each other, or are _in
equilibrium_; but if you disturb this balance, for example, by sucking
the air from a tube closed at one end, it will cling to your tongue;
or if you take a thick piece of leather under ordinary conditions it
will not adhere to anything, but if it be thoroughly wet and pressed
hard against the surface of a smooth stone, so as to force out the air
from under it, the stone, as nearly all school-boys know, can be lifted
up if a string is attached to the leather; or if the air be sucked
out of a tube, one end of which is inserted in a liquid, the latter
will be forced up the tube. These phenomena are due to the pressure of
the atmosphere in the first case on one side of the person’s tongue,
pressing it against the mouth of the tube; in the second, to the same
pressure on the top of the leather, causing it to adhere to the stone;
and in the last, to the weight of the air pressing on the surface of
the liquid, forcing it into the vacuum in the tube.
[Illustration: _Fig. 7._
Scale ¹⁄₄.]
QUESTION 10. _What is the amount of the pressure of the atmosphere, and
how is it measured?_
_Answer._ It is usually measured by the pressure on one square inch of
surface, which at the earth’s surface is 15 pounds.[3] If, for example,
we have a cylinder, _A_, fig. 7, with an air-tight piston, _B_, fitted
to it whose area is just one square inch, if we exhaust the air through
the tube _C_ from the cylinder above the piston, the air will press
against the under side of the piston so that, if no power is required
to overcome its friction in the cylinder, the pressure of the air will
raise a weight of 15 pounds. The pressure of the air varies, however,
as you ascend or descend from the surface of the earth, because as
you go up on a mountain or in a balloon the layer of air above you
becomes thinner, and, therefore, its weight and consequent pressure
are diminished; and as you descend, as in a deep mine, the layer is
thicker, and its pressure consequently greater.
[3] In common practice it is generally taken at 15 lbs. per square
inch, but the average atmospheric pressure is more accurately 14.7
pounds.
QUESTION 11. _What is steam?_
_Answer._ Steam is water changed by means of heat into a gas. At every
temperature there is formed from water, on its surface, vapor of which
the clouds are formed at all seasons of the year. This change of water
into vapor, or evaporation of water, takes place at low temperatures
only on its surface, however. But if we heat water in a vessel to a
temperature of 212 degrees Fahrenheit, then the inner particles of the
mass of water (lying on the heating surface of the vessel) are changed
into steam, and rise to the surface in bubbles, which is the phenomenon
we call _boiling_. It must not be imagined, however, that the visible
cloud which escapes from a kettle or the exhaust-pipe of a steam engine
is true steam. It is rather small particles of water, into which the
steam has condensed through contact with the cold air. True steam
is invisible, as we may observe near the mouth of a kettle or the
exhaust-pipe of an engine from which we know it is escaping.
QUESTION 12. _If water is heated in an open vessel what occurs?_
_Answer._ It continues for some time to increase in temperature, and
the evaporation becomes more and more rapid. At length bubbles of
vapor break out and reach the surface, and the process of boiling or
ebullition has begun. When this takes place the temperature of the
water ceases to rise, and it remains stationary until all the water
has boiled away, the only difference being that if the supply of heat
be very great the process is very rapid, and if the supply of heat be
small the process is very slow. The point at which ebullition commences
is called the _boiling-point_.
QUESTION 13. _On what does the boiling-point depend?_
_Answer._ Chiefly on the pressure on the surface of the water, but
to some extent upon the purity of the water. Thus, boiling, which
takes place at 212 degrees under the ordinary atmospheric pressure,
in lighter air, as on high mountains, takes place at a much lower
temperature than on lowlands, and so water boils in a glass tube from
which the air has been exhausted by the warmth of the hand, that is, at
92 degrees.
QUESTION 14. _What is the pressure of steam which escapes from boiling
water in an open vessel?_
_Answer._ It is exactly equal to the pressure of the atmosphere in
which it is boiled. Ordinarily this is 15 lbs., and the boiling-point
212 degrees; but if we go up on a mountain where the atmospheric
pressure is only 10 lbs. per square inch, the water will then boil at
a temperature of 193.3 degrees, and the steam which escapes will have
the same pressure as the atmosphere, or 10 lbs. per square inch. On
the other hand, if we could go down into a mine where the atmospheric
pressure was 20 lbs. per square inch, the water would not boil until it
was heated to 228 degrees, and the pressure of the escaping steam would
then be 20 lbs. per square inch.
QUESTION 15. _If water is boiled in an enclosed vessel like a covered
tea-kettle or a steam boiler, what occurs?_
_Answer._ The steam rises and fills the space above the water, and, if
it cannot escape, increases in pressure. The temperature of both the
water and the steam rises with the pressure, and will continue to do so
as long as the heat is increased, or until the steam can escape or the
vessel is exploded. The boiling point also rises as the steam pressure
increases.
QUESTION 16. _Is there any pressure which corresponds to the
temperature of steam and water?_
_Answer._ Yes. There is a fixed pressure for every temperature, when
steam is in contact with water, and its pressure cannot be increased
or diminished without at the same time heating or cooling the water,
and the higher the temperature of the water the greater will be the
corresponding steam pressure. Thus water at 212 degrees produces
steam with a pressure equal to that of the atmosphere; at 240 degrees
the steam will have a pressure of 25 lbs., or 10 lbs. more than the
atmospheric pressure; at 281 degrees a pressure of 50 lbs.; and at 328
degrees, 100 lbs. As this relation of pressure to temperature is fixed,
if we know the one we can tell the other. This is true, however, only
where the steam is in contact with water, when it is called _saturated
steam_. If it is separated from water it may be heated to a higher
temperature, and is then called _superheated steam_.
QUESTION 17. _How is the pressure of steam measured?_
_Answer._ In the same way as that of the atmosphere,--that is, by the
force exerted on one square inch of surface. Thus if steam is admitted
into the cylinder _A_, fig. 8, under the piston _B_, whose area is
equal to one square inch of surface, supposing, as we did before, that
no power is required to overcome its friction in the cylinder, if the
steam thus admitted would just balance the atmosphere, its pressure
would be equal to 15 lbs. If, besides overcoming the pressure of the
atmosphere, it would raise a weight of 15 lbs., then its pressure per
square inch would be equal to 30 lbs. When the atmospheric pressure
is _included_ with that of the steam, we call it the _absolute steam
pressure_. In ordinary engines, however, the steam must always overcome
the pressure of the atmosphere, and therefore the only part of the
pressure which is effective is that above, or by which it exceeds, the
atmospheric pressure. For example, although the steam admitted under
the piston in fig. 8 has an absolute pressure of 30 lbs. per square
inch, yet it will only raise a weight of 15 lbs., because it must first
overcome the pressure of the air on the other side of the piston. The
pressure of the steam used in most stationary and in locomotive engines
is, therefore, measured by its pressure above the atmosphere. That is,
if steam introduced under the piston in fig. 8 will raise a weight of
only 15 lbs., we say it has a pressure of 15 lbs. per square inch; if
it will raise 50 lbs., its pressure is said to be 50 lbs. per square
inch, and so on. The pressure of the atmosphere is disregarded, and all
steam-gauges used on locomotives are graduated in that way. In speaking
of steam pressure in future, therefore, unless otherwise specified, we
shall mean _effective_ and not _absolute_ pressure.
[Illustration: _Fig. 8._
Scale ¹⁄₄.]
QUESTION 18. _What is meant by the expansion of steam?_
_Answer._ In all gases a repulsion is exerted between the various
particles, so that any gas, however small in quantity, will always fill
the vessel in which it is held. Steam possesses this same property, and
if placed in any vessel the particles in endeavoring to separate from
each other will exert a force on all its sides. This force we call the
steam pressure. To illustrate this we will suppose that the cylinder
_A_ in fig. 8 is half filled with steam of 30 lbs. pressure. If now the
supply of steam is shut off, the steam in the cylinder will expand so
as to push the piston upward, but with a somewhat diminishing force,
the nature of which we will explain hereafter.
QUESTION 19. _What is meant by the volume of steam?_
_Answer._ It means the space which the steam occupies.
QUESTION 20. _What is the proportion which exists between the volume
and the pressure of steam?_
_Answer._ If the temperatures remain the same they are INVERSELY
PROPORTIONAL TO EACH OTHER; that is, the one increases in the same
proportion as the other diminishes. If we admit steam of 30 lbs.
pressure per square inch into the cylinder _A_, fig. 8, and then cut
off the supply by closing the cock _C_ and allow the steam in the
cylinder to expand to double its volume by pushing the piston to the
end of the cylinder, the steam pressure will then be only 15 lbs.; if
it should expand to three times its volume its pressure would be only
one-third, or 10 lbs. per square inch. This method for calculating
the pressure of steam after it has expanded is correct only for the
_absolute_ and not for the _effective_ pressures of steam. In order
to ascertain the effective pressures of steam after expansion, it is
only necessary to make the calculation with the absolute pressure and
deduct the atmospheric pressure from the result. If, after being thus
expanded, the piston be pushed down again so as to compress the steam
into its original space, its pressure will again be 30 lbs., providing
no heat has been lost in any way.
QUESTION 21. _With a cylinder of any given stroke_[4] _how can we
determine approximately the pressure of the steam after expansion for
any given point of cut-off?_[5]
[4] The _stroke_ of a piston is the distance it moves in the
cylinder, and in ordinary engines is always twice the length of the
crank measured from center to center of the shaft and crank-pin.
[5] The steam is said to be _cut off_ when the steam-port by which
steam is admitted to the cylinder is closed by the valve.
_Answer._ BY MULTIPLYING THE ABSOLUTE PRESSURE PER SQUARE INCH OF
THE STEAM IN THE CYLINDER BEFORE IT IS CUT OFF, BY THE DISTANCE FROM
THE BEGINNING OF THE STROKE AT WHICH IT IS CUT OFF, AND DIVIDING THE
PRODUCT BY THE WHOLE LENGTH OF THE STROKE. Thus, if we have a cylinder
whose piston has a stroke of 24 inches, if we cut off the steam at 8
inches, and have an ABSOLUTE pressure of 90 lbs. in the cylinder, the
calculation is as follows:
90 × 8
------ = 30 lbs. final pressure.
24
If we cut off at 10, 12 and 15 inches, the final pressure would be
37¹⁄₂, 50 and 56¹⁄₄ lbs., respectively. To get the effective pressure
deduct the atmospheric pressure from this result.
QUESTION 22. _What is the proportion between the volume of steam and
that of the water from which it is formed?_
_Answer._ At the pressure of the atmosphere (15 lbs.) each cubic
inch of water will make 1,610 cubic inches of steam. At double that
pressure, or 30 lbs. absolute pressure, it will make a little more than
half as much, or 838 cubic inches; at four times, or 60 lbs. absolute
pressure, 437 cubic inches, or a little more than a fourth as much as
at the pressure of the atmosphere.
QUESTION 23. _Why is it that the quantity of steam at high pressures is
somewhat greater than in inverse proportion to the pressure?_
_Answer._ Because the boiling-point of water, as has already been
explained, is higher as the pressure increases, and therefore the
temperature of the steam produced at such pressure is also higher than
at lower pressures; and as all gases are expanded by heat, therefore
the volume of steam at the higher pressures is somewhat greater than
in inverse proportion to its pressure, on account of being somewhat
expanded by its high temperature. To make this plain, if we take a
cubic inch of water and convert it into steam of atmospheric pressure,
its volume will be 1,610 times that of the water, and its temperature
212 degrees.[6] If we convert this quantity of water into steam with a
pressure double that of the atmosphere, the volume of the steam will be
838 times that of the water and its temperature will be 250.4 degrees.
If the volume of the steam were exactly _inversely proportional_ to the
pressure, the cubic inch of water at double the atmospheric pressure
would make only 805 cubic inches of steam; but as the boiling-point at
that pressure is 38.4 degrees higher, the steam is expanded 33 cubic
inches by the increase of its heat due to the higher boiling-point.
[6] More accurately, 213.1 degrees, if we call the atmospheric
pressure 15 lbs., as we have.
A table in the appendix gives the pressure, temperature and volume of
saturated steam up to 300 lbs. absolute pressure.
QUESTION 24. _What is meant by the condensation of steam?_
_Answer._ It is the reconversion of steam into water by cooling it,
or depriving it of part of its heat. It has been shown that the
temperature of water must be raised to a certain point to generate
steam of a given pressure. If the process is reversed, and we deprive
the steam of a part of its heat, some of the steam is then at once
reconverted into water, or _condensed_, and the pressure of that which
remains will be reduced just in proportion as the heat is lost. When
the temperature gets below 212 degrees under atmospheric pressure,
all the steam will be condensed. As the useful work which steam can
do in an engine is due to its pressure, which in turn depends on its
temperature, any loss of heat will diminish its effective power. For
this reason, all waste of heat from a steam engine should, as far as
possible, be prevented.
QUESTION 25. _How is the heat of the steam wasted or lost in an
ordinary steam engine?_
_Answer._ It is wasted in three ways: first, by _conduction_; second,
by _convection_; and third, by _radiation_.
QUESTION 26. _What is meant by these three terms?_
_Answer._ 1. By _conduction_ is meant that phenomenon which is
manifested when we put one end of a metal bar two or three feet long
into the fire and heat it. The heat is then gradually conveyed from one
particle of the metal to that next to it until finally the end of the
bar farthest from the fire becomes so hot that it cannot be touched.
The heat is then said to be _conducted_ through the bar. In the same
way the metal of the boiler, pipes, cylinders and other parts of the
engine becomes heated on one side, and the heat is thus conveyed to the
outside of these parts.
2. The air with which they are surrounded then becomes heated, and
being then lighter than the cold air, it rises and is again replaced
with air which is not heated. In this way the heat is conveyed away by
the air, and this phenomena is therefore called _convection_.
3. If an iron plate be placed in front of an ordinary grate fire three
or four feet from it and exposed to the rays of heat from the fire, it
will soon become so hot that you cannot bear your hand on it. If you
place your hand between the iron plate and the fire you will find that
only the side of your hand which is exposed to the fire will become
hot, showing that the air between the plate and the fire is not nearly
so hot as the plate soon becomes, and therefore that the heat is not
conveyed to the plate by the air between it and the fire, but by the
rays from the fire. This phenomenon is called _radiation_. The same
thing occurs from any hot body, as for example a coil of steam pipe for
heating a room, a steam boiler or cylinder of an engine.
QUESTION 27. _Is there any difference in the conducting and radiating
power of different substances?_
_Answer._ Yes, very great. The difference in the _conducting_ power
of wood and iron is shown if we place one end of a bar of each in the
fire. The wood will be consumed without warming the bar more than a
few inches from the fire, whereas the iron will soon become hot two or
three feet from the fire. Owing to the difference in the conducting
power of cotton and wool, we wear cotton clothing in summer and woolen
in winter, because cotton allows the heat of the body to be conducted
away from it, whereas woolen cloth prevents to a great degree this
loss of heat. For the same reason, the venders of roasted chestnuts
on our streets wrap them in a piece of blanket to keep them hot, that
is, to keep the heat in; and in summer we wrap ice in the same way to
keep it cold, that is, keep the warmth of the air out. The wool, being
a very bad conductor of heat, simply prevents the heat from being
transferred from the inside to the outside, and _vice versa_. It is for
this reason that steam boilers, pipes and cylinders are nearly always
covered with wood, and sometimes with felt.
The difference in the _radiating_ power of various substances can be
shown if we take a large thermometer and heat it up to the temperature
of boiling water. If this thermometer is hung up in a room having the
temperature of melting ice, it will lose heat in two ways,--first by
heating the air which surrounds it, that is by _convection_, and also
by _radiation_. In order to confine ourselves to the latter process, we
will suppose that the chamber is a vacuum. If we first cover the bulb
of the thermometer with a thin coating of polished silver, and then
ascertain how much heat it radiates in a minute, and then coat it with
lamp-black, and repeat the same experiment,--that is to say, allow the
thermometer at the boiling-point to cool for one minute in a vacuum
chamber at the freezing-point,--it will be found that the thermometer
loses much more in a minute when coated with lamp-black than it did
when coated with silver, showing that much more heat is radiated from a
surface covered with lamp-black than from polished silver. Generally it
may be stated that polished metals radiate much less heat than surfaces
which are not polished.[7] For this reason, as well as for ornament,
locomotive and other boilers and cylinders are usually covered with
Russia iron or polished brass.
[7] The account of the above experiment is copied from Balfour
Stewart’s very excellent little book, “Lessons in Elementary
Physics,” of which, and the same author’s “Elementary Treatise on
Heat,” the writer has made frequent use.